Re: [Haskell-beginners] How best to do this?

29 Apr 2013

      ROFLCOPTER indeed Batman!

I had no idea of that... I just *assumed* (usual rules apply I guess) that
[3..0] was the "opposite" of [0..3] but sure enough a wuicj ghci session
reveals the bitter truth!

Thanks again... i can see that it's not just me that is too lazy at times.
I guess writing [3,2..0] will do for now but is that a bug or is there some
other reasoning behind it?

We live and learn, well, I live anyway...

:)

On 29 April 2013 16:37, David McBride  wrote:
...
One other gotcha.  I don't know why it is this way, but [3..0]
evaluates to [].  I have no idea why reverse notation is not allowed.
But you can just manually reverse it or you can go [3,2..0].
...
damn that lazy evaluation! LMAO  ...a good point brent and yuo have no
doubt
saved me hours of head scratching this evening when I try out the "new
improved software". Oh dear oh dear oh dear...
doOption dev (Forward n)  = do
  putStrLn $ "> STEP FORWARD " ++ (show n)
  stepBits dev ioPORTA [3..0]
doOption dev (Backward n) = do
  putStrLn $ "> STEP BACKWARD " ++ (show n)
  stepBits dev ioPORTA [0..3]
stepBits dev port = mapM_ stepIt
  where stepIt bit = mapM_ (\s -> HW.setPortBit dev port bit s >>
stepDelay)
[0,1]
I now have the above as my current "final" implementation... hopefully
On Mon, Apr 29, 2013 at 11:26 AM, emacstheviking 
wrote:
that
...
*does* do what I think it does because mapM_ is driving it and will cause
evaluation of the actions?
On 29 April 2013 15:56, Brent Yorgey  wrote:
...
On Mon, Apr 29, 2013 at 02:59:29PM +0100, emacstheviking wrote:
...
I have built a library for using the Hexwax expandIO-USB chip and I
have
...
...
now got some code to drive a stepper motor:
doOption :: HWHandle -> Flag -> IO ()
doOption dev (Backward n) = do
  putStrLn $ "> STEP BACKWARD " ++ (show n)
  let x = [ stepBit b | b <- [3..0]]
  return ()
    where
      stepBit p b = setBit p b 0 >> setBit p b 1
        where setBit p b s = HW.setPortBit dev p b s >> stepDelay
The other posted solutions are good, but I also want to make a very
important comment about the above code: it does not actually step any
bits!  All it does is print some stuff.  x is simply a name for a list
of IO actions; it is never used so it just gets garbage collected and
the IO actions are never run.
-Brent
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